Porous defective carbon ferrite for adsorption and photocatalysis toward nitrogen compounds in pre-treated biogas slurry

Carbon ferrite (C-Fe3O4) with hydrophilic functional groups and lattice defects was synthesized in anhydrous molten alkali system by fern leaves and ferric chloride as raw materials. Structural characterization results showed that carbon ferrite obtained oxygen-containing groups on the carbon surface. And structural pores and lattice defects resulted from spontaneous accumulation and “directive-connection” of ferrite (Fe3O4) nanoparticles. Carbon ferrite displayed an adsorption efficiency of 29.0% and excellent photocatalytic degradation of 80.8% toward nitrogen compounds (initial concentration of 430 mg/L) in pre-treated biogas slurry. The micromechanism for nitrogen compounds removal was discussed at the molecular/atomic level by exploring carbon ferrite “structure-activity”, which provides a design idea from microscopic perspective for the preparation of environmental materials with reactive sites.

. CeO 2 obtained in organic solvents increased the adsorption capacity due to rich lattice defects 15 . Structural defects of MoS 2 improved photocatalytic degradation activity by promoting charge separation, and active lattices of TiO 2 could facilitate charge carrier scattering as an electronic driving force [16][17][18] , but their micromechanisms have rarely been explored. In this paper, by melted double bases as a reaction solvent without Ostwald ripening [19][20][21] , carbon ferrite (C-Fe 3 O 4 ) was synthesized with fern leaves and iron trichloride as reaction materials, and ferrite nanoparticles formed cumulate holes and lattice defects by "directive connection". Carbon ferrite showed synergistic effects by carbon adsorption and ferrite photocatalytic degradation toward NC in pre-treated biogas slurry, and their mechanisms have also been revealed at the microscopic level. In addition, magnetic ferrite can be used to separate carbon ferrite composites from wastewater for recycling.

Results
Morphology and microstructure. The SEM image of the carbon ferrite sample illustrated in Fig. 1a indicates tablet carbon with granular ferrite nanoparticles. The TEM image in Fig. 1b     The nitrogen adsorption-desorption isotherm of carbon ferrite is a type IV isotherm with a typical hysteresis in Fig. 3. The hysteresis loop is a mixed type of H1 and H3, and the hysteresis loops of H1 and H3 formed by aggregations of uniform nanoparticles and flaky grains, respectively. SEM and TEM characterizations results indicated that the porous structures were produced by the coaggregation of ferrite nanoparticles and carbon nanosheets. The pore size distribution in the inset of Fig. 3 suggests that carbon ferrite had pore sizes from micropores to mesopores due to "close oriented attachment" of ferrite nanocrystals and carbon nano-blocks corrosion. Total pore volume of carbon ferrite was about 0.048 m 3 /g in Table 1, which is smaller than that of porous defective oxides prepared by the anhydrous system 14,15 , however, but the diameter of ammonia is only 0.5 nm. The micropores diameter and pore volume of carbon ferrite matched well the size of conical ammonia/ ammonium, and NC in pre-treated biogas slurry could be adsorbed effectively and photo-degraded subsequently by carbon ferrite materials 4 .
Surface and optical properties. The FT-IR spectrum of the carbon ferrite sample (Fig. 4) displays two characteristic peaks at 1100 and approximately 3400 cm −1 corresponding to the stretching mode of the OH groups. The bands at 1720 and 1730 cm −1 on the carbon surface demonstrate the vibrations of COOH groups, which assuredly means carbon ferrite has obtained oxygen-containing functional groups as fern leaves contain bioactive and optically active carboxyl-containing alkalis 31 . The Raman spectra showed the microstructure of fern-leaf carbon prepared by molten alkali method in Fig. 5. The 1327 and 1576 cm -1 features come from D and G bands, and the broad peak at 2550 cm -1 located in 2D-bank region. The D band corresponds to structural defects and G band is allocated to olefinic carbon structure 32,33 . Fern-leaf carbon obtained defective surface by base etching and oxygen-containing functional groups from fern leaf raw materials. The thermal stability and the component contents of carbon ferrite sample without pre-calcination were investigated by TG analysis as showed in Fig. 6. Weight loss of 6.008 mg was observed before 250 ℃ duo to the detachment of adsorbed water and fern-leaf carbon precursor decomposition. Fern leaf carbon was completed oxidized by oxygen and hydroxyl iron oxides changed to iron oxides between 250 and 730 ℃, resulting in 7.5 mg weight loss. After 730 ℃, weight change of the sample was attributed to the iron oxide transition. The final content of iron oxide in carbon ferrite composites was 67.1% and iron oxide displayed good thermal stability before 1500 ℃.      www.nature.com/scientificreports/ Adsorption and photocatalytic activity. The structural activity of the samples was tested by adsorption and photocatalysis toward NC in pre-treated biogas slurry. The reaction process can be expressed as a pseudo-first-order kinetic equation: ln(C 0 /C) = kt, where C 0 /C is the normalized NC concentration and k is the apparent reaction rate (min -1 ), and the sample activity has been defined as the corresponding reaction rate constant summarized in Table 2 26 36.0%, 21.7% and 29.0% NC were removed by fern-leaf carbon, iron oxide and carbon ferrite after adsorption in the dark, respectively (Fig. 8a, 0-120 min). Fern-leaf carbon is greater than iron oxide in adsorption capacity due to surface functional groups interacting with NC and abundant porosity by melted base etching, which is consistent with the k constants of the adsorption process (Fig. 8b, 0-120 min). The 60.2%, 67.1% and 80.8% NC were decomposed by fern-leaf carbon, iron oxide and carbon ferrite after visible light illumination (Fig. 8a, 120-240 min). The k constant (0.0108 min -1 ) for carbon ferrite is much higher than those for fern-leaf carbon (0.0039 min -1 ) and iron oxide (0.0073 min -1 ) (Fig. 8b, 120-240 min). Carbon ferrite sample appeared more prominent effect on NC removal than fern-leaf carbon and iron oxide in term of photocatalytic degradation efficiency. The circularizability of carbon ferrite sample was examined in Table 1. NC removal decreased by less than fifteen percent after the second usage and kept more than half of the original   Nitrogen compounds removal mechanism. Figure 9 exhibits the electronic structures of ammonium ion. Nitrogen atom of ammonium ion has a solitary pair of electrons and the iron atoms of iron oxide have 3d empty orbits, and the solitary pair of electrons could spread easily to the 3d free orbits of iron atoms to form a close connection. The H-bonds were produced by electrostatic interaction between the oxygen-bearing groups (-COOH, -OH) on the carbon surface and hydrogen ions of ammonium ions. NC pollutant molecules would been adsorbed by the pores physical function and active-sites chemical interaction. Fern-leaf carbon had high adsorption efficiency but low photocatalytic efficiency. Ferrite (Fe 3 O 4 ) showed outstanding photocatalytic activity through guided-linking mismatched lattices (Fig. 1b) into the microscopic channels for transferring photoelectrons (Fig. 10) 18 . The microbial dissimilatory reduction process between microorganisms in pre-treated biogas slurry and ferrite formed "microbial-Fe(III)-N" interactions, as shown in Fig. 10. Therefore, carbon ferrite revealed a collaborative cycle effect on the removal of NC in pre-treated biogas slurry by carbon adsorption and ferrite degradation. This provides an idea for designing highly active photocatalysts by the main carbon ferrite structure-activity relationship and NC removal mechanism, which are both explored at the microscopic level.  www.nature.com/scientificreports/ Concluding remarks. Carbon ferrite with accumulated pores and structural defects was synthesized by the melted alkali method. Functional groups on the surface of carbon improved water dispersibility and absorptivity, and lattice defects of ferrite increased reactive sites and microscopic channels for the photocarriers. Carbon ferrite displayed an adsorption efficiency of 29.0% and a degradation efficiency of 80.8% toward NC in pre-treated biogas slurry by carbon adsorbing, ferrite photodegradation and "microbial-Fe(III)-N" interactions. The research findings have positive significance to reduce nitrogen pollution in water and soil environments and promote the healthy development of livestock breeding.

Materials and methods
Mixed sodium hydroxide and potassium hydroxide (AR, Jingke Chemical, Casma) were used as reaction solvents, and ferric chloride (AR, Xiongda Chemical, Casma) and fern leaf powder were treated by smashing and grinding ferns as raw materials. Carbon ferrite was obtained by the melted mixed bases method. One hundred grams of mixed hydroxides (mass ratio of NaOH/KOH = 1:1) were preheated to 180 ℃ for melting in a reaction kettle with a PTFE inner tank of 200 ml. And 28 g fern leaf powder with a mean diameter of 0.015 cm and 16.25 g ferric chloride were added to the molten hydroxides. The solid products were obtained after the 12 h reaction at 180 ℃ under normal pressure, washing using hydrochloric acid solution of pH = 1 and distilled water to neutral, and 2 h calcination at 500 ℃ in a N 2 atmosphere. Fern-leaf carbon and iron oxide were synthesized separately via the melted bases method for comparison experiments.
The synthetic samples were characterized by X-ray powder diffraction (Philips PW3040/60), scanning electron microscopy (Hitachi S4800, Japan), transmission electron microscopy (JEOL 2100 F, Japan), Fourier transform infrared spectrometry (RAYLEIGH WQF-510A) at wavenumbers of 3800-600 cm −1 , Raman spectrometer with laser wavelength of 532 nm (Labram HR Evolution, Horiba), and thermal weight analyzer (TGA/DSC 2 STAR System, Mettler Toledo) with a heating rate of 15 ℃/min in the oxygen atmosphere. N 2 isotherms were recorded on a Quantachrome NOVA 2000e sorption analyzer, and the pore size distribution was based on the Barrett-Joyner-Halenda Model.
The initial concentration of nitrogen-containing compounds (NC) in biogas slurry from a local pig farm was 2800 mg/L. Biogas slurry was treated with bacterial carbon purification agent and the remaining NC (ammonium/free ammonia) concentration of pre-treated biogas slurry was 430 mg/L. The structural activity of synthetic samples was evaluated by the removal of NC in pre-treated biogas slurry with pH of 5-6. 800 mg of the sample and 800 ml of pre-treated biogas slurry (1 mg:1 ml) were placed in a 1000 ml tubular quartz reactor with a cooling water setting and irradiated by a 200 W lamp bulb as an illuminant (emission wavelength λ max = 550 nm). Fifty milliliters of react liquid was sampled and measured at 30 min intervals by alkaline K 2 S 2 O 8 Colorimetry (GB 11,894-89) on an SP-722 spectrometer for absorbance. Carbon ferrite photocatalyst was reactivated by washing in 2 M sodium hydroxide solution with stirring magnetically for 2 h. The recycling of carbon ferrite sample was investigated by reusing carbon ferrite powder separated magnetically from wastewater for adsorption and photocatalytic experiments as shown above.

Data availability
All data generated or analyzed during this study are included in this manuscript, and the datasets used and analyzed during the current study are available from the corresponding author on reasonable request.